Method and apparatus for centrifugally separating finely divided solids from aqueous suspensions thereof

ABSTRACT

Finely divided solids are centrifugally separated from an aqueous suspension thereof by forming a flowing sheet of the aqueous suspension, preferably a hollow swirling cone of the aqueous suspension, spraying a solution of a water soluble, high molecular weight, long chain flocculant substantially at right angles into a side face of the flowing sheet so that flocculant is absorbed into wetted surfaces of the finely divided solids, without totally wrapping individual finely divided solids by the flocculant and without entrapping finely divided solids by premature bridging, flocculating the finely divided solids to form dense, strong flocs predominantly larger than the &#39;&#39;&#39;&#39;size cutpoint&#39;&#39;&#39;&#39; of a cyclone separator, and then centrifugally separating a major portion of the dense, strong flocs from the aqueous medium by means of that cyclone separator.

United States Patent 1 Visman et a1.

[ Dec. 30, 1975 [75] Inventors: Jan Visman, Sydney, Canada;

Hassan A. Hamza, Cairo, Egypt [73] Assignee: Canadian Patents and Development Limited, Ottawa, Canada [22] Filed: Feb. 28, 1974 [21] Appl. No: 446,903

[30] Foreign Application Priority Data Mar. 29, 1973 Canada 167482 [52] US. Cl. 210/49; 210/84; 210/209; 210/512; 259/4; 261/76; 26l/DIG. 75

[51] Int. Cl. B011) 211/26 [58] Field of Search 210/51, 52,54, 78, 84, 210/199, 202, 207, 209, 259, 294, 512, 512

M, 59, 49; 209/144, 211; 261/76, DIG. 75;

259/4, DIG. 30

[56] References Cited UNITED STATES PATENTS 3,572,500 3/1971 Kouloheris 209/5 3,794,299 2/1974 Wagner et a1. 259/4 Primary ExaminerThomas G. Wyse Assistant ExaminerF. F. Calvetti Attorney, Agent, or Firm-Francis W. Lemon [57] ABSTRACT Finely divided solids are centrifugally separated from an aqueous suspension thereof by forming a flowing sheet of the aqueous suspension, preferably a hollow swirling cone of the aqueous suspension, spraying a solution of a water soluble, high molecular weight, long chain flocculant substantially at right angles into a side face of the flowing sheet so that flocculant is absorbed into wetted surfaces of the finely divided solids, without totally wrapping individual finely divided solids by the flocculant and without entrapping finely divided solids by premature bridging, flocculating the finely divided solids to form dense, strong flocs predominantly larger than the size cutpoint of a cyclone separator, and then centrifugally separating a major portion of the dense, strong flocs from the aqueous medium by means of that cyclone separator.

5 Claims, 4 Drawing Figures US atent Dec. 30, 1975 SheetlofZ 3 29,33

FLOCCULATOR US. Patent Dec. 30, 1975 Sheet 2 of2 3,929,633

METHOD AND APPARATUS FOR CENTRIFUGALLY SEPARATING FINELY DIVIDED SOLIDS FROM AQUEOUS SUSPENSIONS THEREOF This invention relates to a method and apparatus for centrifugally separating finely divided solids from aqueous suspensions thereof.

Presently, centrifugal separation can be achieved by centrifuges of different design e.g. basket centrifuges, in which flocculants may be used. These machines act as filters through which the centrate is forced by cen trifugal acceleration.

Another centrifugal separator used is the decanter whose products are improved by applying flocculants during decantation.

Other centrifugal devices may be constructed to perform similar operation e.g. a device in which centrifugal separation is effected by mechanical means, e.g. rotating impeller and an arrangement of stationary vanes for deflecting and the flocs and thereby causing them to substantially separate the flocs from the liquid.

Cyclones belong to the category of centrifugal separators. This invention relates to a method and apparatus for cyclonic separation of fine solids suspended in liquids. The principle involved is forming strong, compact flocs of such strength as to withstand the shear forces imposed on them in a centrifugal separator. This is achieved by flocculation in a flocculator which design depends on the two principles of quick dispersion of the flocculant in the suspension and mild agitation to promote growth of the floc size.

Cyclonic separators can be used for the stripping of solids from a effluent at greatly enhanced speed owing to centrifugal accelerations many times that of gravity, their small size in relation to capacity, low cost and the absence of moving parts.

Despite the above mentioned advantages, they are presently rarely used industrially for separating effluents containing particles of less than micron equivalent diameter because, in order to separate such very fine particles from the liquid small-bore, long-tapered cyclones must be used, with their inherent disadvantages such as, frequent plugging and high maintenance.

For complete separation, a cyclone must have a size cut-point (d 50), well below the diameter of the finest particle in suspension. The size cutpoint is determined from known characteristics of the cyclone and is defined as the size of the largest flocs not substantially separated by the cyclone or, put another way, the size above which the flocs must be to be substantially separated by the cyclone.

The conventional solution for collecting the minus lO-microns particles is, to flocculate them and allow them to settle in static thickeners and clarifiers operating under gravity.

This permits low throughput capacity only and requires considerable space for the equipment. As well, the high cost and large size of conventional separators prevent their use in many instances. At the same time, the need for collecting colloidal material and ultra-fine particles of sub-micron size is increasing with the demand for clean water in all segments of society.

Special methods for collecting these particles, with their enormous specific surface area, are required to effectively settle certain pollutants of lakes and rivers, and the impalpable slimes often encountered in mineral processing and metallurgical plants. The principle of the invention is perhaps best described by the following simile: a strong wall is made by building it brick-bybrick rather than pouring a load of mortar on to a pile of bricks. In the same manner, small strong flocs are made by process of intensive, high-speed interspersion of the effluent and the flocculant, followed by a controlled floc growth.

In order to exploit the greater advantages of separating by means of cyclones it is essential to increase the particle size by flocculation, and increase the shear strength of the flocs to its maximum by a process that permits the most intimate bonding between the flocculant segments and the active sites on the particle sur face to be established.

Flocculation is effectively achieved by means of longchain flocculant molecules, in three steps; that are characterised by the following phenomena: (In this specification adsorption, bridging and entanglement are defined as follows:)

1. Adsorption: the attachment of a fraction of the total segments (functional groups) of a flocculant molecule on a fraction of the active sites on a particle surface. The remaining segments of the molecule will extend into the suspension.

2. bridging: the linking between two particles by the adsorption of particles on the extended remaining segments of the flocculant molecule or by the interaction between extended segments being adsorbed on different particles.

3. entanglement: the reinforcing and densifying of the formed floc by strengthening particle-to-particle bond within the floc by means of increasing the number of particle-to-flocculant attachement. Further strengthening is also achieved by mechanical entanglement of the flocculant filaments. This is done through mild agitation.

Floc strength is achieved by rapidly and throughly dispersing the flocculant in the suspension in such a way as to promote maximum bridging rather than excessive adsorption, that is rather than wrapping flocculant polymer around individual particles.

Floc size is determined by flocculant dosage and the time for floculation. Dosage in two or more stages increases the floc density and floc strength as it affords better control of the balance between bridging and adsorption or, put in a somewhat different way, the quality of the flocs are enhanced by repeating the interspersion mild agitation cycle. Briefly summarised,

the flocs thus formed permit the effluent to be effec-' tively separated by cyclones or slugging cyclones of commercial size.

It is an object of the present invention to provide a method and apparatus for centrifugally separating finely divided solids, from aqueous suspensions thereof, wherein the finely divided solids are flocculated to form dense, strong flocs which are predominantly larger than the size cutpoint of a cyclone separator, so that a major portion of the flocs may be separated by that cyclone separator.

According to the present invention there is provided a method of centrifugally separating finely divided angles into a side face of the flowing sheet so that the flocculant is adsorbed onto the wetted surfaces of the finely divided solids without totally wrapping individual finely divided solids by the flocculant molecules and without entrapping finely divided solids by premature thereof, comprising a blender, a flocculator connected to the outletof the blender, and a cyclone separator connected to the outlet of the flocculator, and wherein the blender comprises a cylindrical casing with an aqueous suspension inlet for directing aqueous suspension into the casing to form a rotating swirling hollow conetherein and an outlet for conveying the rotational flow substantially tangentially out of the casing and at a position spaced along the length of the casing from the inlet, .an annular. web secured inside the casing and extending radially inwardly between the inlet and outlet, a vortex finder disposed coaxially within the casing and sealed, at. an intermediate web extending beyond the inlet and outlet, a flocculant solution nozzle in the casing and facing the end of the vortex finder extending beyond the outlet, the nozzle being shaped to direct a spray of flocculant solution substantially at rightangles. into the. hollowcone of aqueous suspension in the vor-" tex finder and passing from the inlet to the outlet.

Preferably the finely divided solids are flocculated to a size far greater than microns.

In the accompanying drawings which illustrate, by way of example, an embodiment of the present invention,

FIG..1 is a schematic view of an apparatus for separating finely divided solids from an aqueous suspension thereof,

FIG. 2 is a schematic view of a flocculator,

FIG. 3 isan enlarged plan view of a blender shown in FIG. 2 and FIG. 4 is a section side view along IV'IV, FIG. 3.

In FIG. 1 there is shown a storage container 1 for an aqueous suspension 2 of finely divided solids, having a suspension inlet 3 and outlet 4. A pump 6 is connected to the outlet 4 for delivering suspension, from the con-.

tainer' l, by pipe 7 to a flocculator-8.

A flocculant storage bin 10 is shown containinga water-soluble, higbmolecular-weight, long chain flocculant 12 in powder form or as a thick liquid, and has a flocculant feeder outlet 14 for feeding flocculant solution outlet pipe 22 leading to the flocculator 8.-The flocculator 8 has an outlet 24 leading to a cyclone separator 26 which has an outlet 28, for densified suspension, leading to a centrifugal separator 30. The cyclone separator 26 has another outlet 32, for separated water and remnants of flocs, leading to a second separator 34. The second cyclone separator 34 has an outlet 36,for densified suspension, leading to the centrifugal separator 30 .and a clarified water outlet 38.

In FIG. 2 the flocculator is shown comprising a first cyclonic blender 40 connected to the aqueous suspension pipe 7 (FIG. 1) and the flocculant solution, pipe 22 via a booster pump 44 and inlet pipe 46. The cyclone blender 40 has an outlet-48 leading to a first flocculator pipe 50. The first flocculator pipe 50 has an outlet 52 leading to a second cyclone blender 54. The cyclone blender 54 has a flocculantsolution inlet 56, branching off from pipe 46, and'an outlet 58 leading to a second flocculator pipe 60. q

The second flocculator pipe 60 hasan outlet 62 lead ing to a pulp divider 64, the pulpdivider returns undersize or very small flocs by pipe 66 to suspension inlet pipe 7, and passes flocculated suspension by outlet 24 to cyclone separator 26. y

The cyclone blenders 40 and 54 are identical and so adescription of the cyclone blender 40, shown inFIGS. 3 and 4 will also apply to the cyclone blender 54.

As shown in FIGS..3 and 4 the cyclone blender 40 comprises. a cylindrical casing 68 having a suspension inlet 70 connected to the pipe 7, (FIGS.. 1 and 2), a spray nozzle assembly 74 connectedtothe flocculant solution pipe 22 and an airinjection nozzle 78.

The spray nozzle assembly -74 is mounted in an end wall 76 of the casing 68, and the air injection nozzle 78 facing the spray nozzle assembly 74 is mounted by a tube80 in a wall 82 of the casing 68 facing an annular web 84, and opposite the end wall 76. The annular web 84 is sealed around its outer edge to extend radially across the interior of the casing ,68 at an intermediate position between the inlet 70 and outlet 48. The air 7 injection nozzle .78 is connected by .a tube 79 to a source 81 of pressurized air. A cylindrical, open ended vortex finder 86 isin the casing 68 and is coaxial within and sealed .to the web 84 to extend, :from each .side thereof to positions, beyond the inlet 70 and outlet 48.

In operation aqueous suspension 2 (FIG. 1) from the container 1 is pumped by pump 6 through pipe 7 into the flocculator 8 whilst flocculant 12 is fed into the mixing'tank 16, mixed with water from inlet 20 by agitator, 18 and fedas a dilute solution to flocculator 8 by pipe22.

The aqueous suspension passingalong pipe 7 enters the blendercasing 68 (FIG. 4) tangentially, rotating as. a swirling hollow cylinder along the inner wall of vortex finder 86, and flowing around rim 85 of the vortex finder 86 .as aswirling hollow cone to discharge from the casing68 by the outlet 48, thus the aqueous suspension forms a thin flowing sheet'in the casing 68. The flocculant solution from pipe 22 (FIG. 2) is pumped by pump 44 and sprayed by nozzle 74 (FIG. 4) as a hollow cone, substantially atright angles into the side of all of theflowing sheet of effluent flowing around the rim 85 so that flocculant and aqueous suspension are rapidly and intensively interspersed. Clearly,for the flocculant to be sprayed into the side face of the flowing sheet of effluent it is necessary for the kinetic energies of the flowing sheet flowing around the rim 85 and that of the sprayed flocculantto be matched. This rapid and intensive interspersion of the flocculant and aqueous suspension causes flocculant to be adsorbed onto the wetted surfaces of the finely divided solids, without totally I wrapping individual finely divided solids by the flocculant molecules and without entrapping finely divided solids by premature bridging by the flocculant molecules. The shape of the swirling hollow cone leaving the rim 85 of the vortex finder 86 in the casing 68 is maintained by pressurizingair within the swirlinghollow cone with air from the nozzle 7850 that the kinetic energies'of the flocculant and that of the aqueous suspension are not dissipated by eddy currents and fluid masses that would otherwise tend to be in the way of the intersection of the'sprayed hollow cone of flocculant and the swirling hollow cone of aqueous suspension which is fed within the swirling hollow cone into the hollow center thereof in the outwardly tapering direction of the swirling hollow cone.

The effluent, containing the finely divided solids with clone separators 26 and 34. The dense, strong flocs in aqueous medium, pass from the flocculator pipe 60 (FIG. 2) to the pulp divider 64 where undersize or very small flocs are returned to'tank l.

flocculant adsorbed on the wetted surfaces of finely 5 The dense, strong flocs, in aqueous medium, of the divided solids leaves the casing 68 by outlet 48 (FIGS. predetermined minimum size pass from the pulp di- 2 and 3) and passes to flocculator pipe 50 where the vider 64 to the cyclone separator 26, where most of the finely divided solids are flocculated by means of the flocs are separated and pass to the centrifugal separator flocculant adsorbed on to the wetted surfaces, to form 30. The aqueous medium containing some flocs passes d ns tr ng flOCS which are larg r than th iz Cutfrom the cyclone clarifier 26 to the cyclone separator point of the cyclone separators 26 and 34. 34, where water separated from the flocs leaves by pipe The dense, strong flocs in aqueous medium pass from 38 and the flocs are passed to the centrifugal separator the flocculator pipe 50 into cyclone blender 54 where 30. The centrifugal separator 30 extracts most of the a swirling hollow cone is formed similar to that in cyremaining water from the flocs to leave then in the clone blender 40 as further flocculant solution is form of a moist cake. sprayed substantially at right angles to a side face of the In some embodiments of the present invention flochollow cone so that further flocculant is adsorbed on to culant solution need only be sprayed once into the the wetted surfaces of the finely divided solids. The aqueous suspension and the suspension with flocculant addition of further flocculant increases the average size adsorbed on to the wetted surfaces of the finely divided of the dense, strong flocs already formed so that the solids need only be flocculated once and centrifugally dense, strong flocs are predominantly larger than the separated once. size cutpoint of the cyclone separators 26 and 34. Thus In some cases the effluent is pre-conditioned either finely divided solids with further flocculant adsorbed magnetically or by a coagulant before it enters the on to the wetted surfaces and in the aqueous medium flocculator.

then pass from the cyclone blender 54 are of a size predominantly larger than the size cutpoint of the cv- The following results were obtained using the apparatus shown in FIGS. 1 to 4.

TABLE I CHARACTERISTICS OF EFFLUENTS AND CORRESPONDING FLOCCULANTS CHARACTERISTICS FLOCCULANT SAMPLE Description Fraction Trade Optimum 25;.|. Name Manufacttotal dos- Wt. urer Type Charge age (lb/t.)*

l. Shaley Coal wash- 74 Super- Cyanamid Polyacryla- Non- Tailings ery flotatfloc 127 mide ionic 0.06

ion tailings (72.5% ash) Sepanan Dow Polyacryl- Anionic 0.1 l

pH: 7.6 MG 200 amide ll. Coal Low ash (3.8%) 59 Nalcolyte Alchem Polyamide Cationic 0.09 Product high sulphur 603 (2.7%) coal 1- from Harbour Separan Dow Polyacryl- Anionic 0.07

Seam, N.S. MG 200 amide pH: 7.8 111. Coal High ash 93 Alum Canada A12 (S04); Cationic 0.10 Product (20%), high Colors &

sulphur (7.8%) Separan Chemicals Polyacryl- Anionic 0.08

Same origin MG 200 Dow amide as 111. pH: 8.0

Pounds of flocculant (as commercially supplied) per ton of dry solids in the suspension.

TABLE II EFFECT OF FLOCCULATION AND CYCLONING ON LOW ASH HIGH SULPHUR COAL FlNES Flocculated feed to Reconstituted sample Size Range Unflocculated the cyclone after cycloning (microns) Midpoint Weight Midpoint Weight Midpoint Weight (microns) Cum. Slice (microns) Cum. Slice (microns) Cum. Slice Plus 250 262.5 100.0 7.5 262.5 100.0 2.5 225 X 250 237.5 92.5 1.4 237.5 97.5 1.5 200 X 225 212.5 91.1 3.9 212.5 96.0 2.0 175 X 200 187.5 87.2 4.8 187.5 94.0 3.5 150 X 175 162.5 82.4 6.7 162.5 90.5 4.8 125 X 150 137.5 75.7 11.4 137.5 85.7 7.7 100 X 125 112.5 64.3 30.4 112.5 78.0 20.6 75 X 100 87.5 33.9 19.3 87.5 57.4 26.7 50 X 75 62.5 14.6 8.8 62.5 30.7 30.7 45 X 50 47.5 100.0 7.2 40 X 45 42.5 92.8 5.3 35 X 40 37.5 87.5 11.8 x 32.5 75.7 16.5 i 25 x 30 27.5 59.2 30.8 (A m 20 X 25 22.5 28.4 8.9 a l g g 15 X 20 17.5 19.5 3.6 10 X 15 12.5 15.9 2.8

Mean: 27.3 Mean: 124.9 Mean: 1 Variance: 148.5 Variance: 3470.6 Variance: 2639.4 Std. Dev.: 12.2 Std. Dev.: 58.9 Std. D -1 TABLE III EFFECT OF FLOCCULATION AND CYCLONING ON A HIGH ASH HIGH SULPHUR COAL Flocculated feed to Reconstituted sample Unflocculated the cyclone after cycloning Midpoint Weight Midpoint Weight Midpoint Weight (microns) Cum. Slice (microns) Cum. Slice (microns) Cum. Slice Size Range (microns) 550050500 6 055558282 4 6 6 c I 6 I 235697 20 4759M7 fi l e 11 1 1 P .m 2 mm m 6.. mm 5 i e 0 50 0 0 500 l7 sn m 005050202 I2 08638 258 flo 2 w u 0 87 6 4 94 Fo 2 099988752 W C 099998642 l l c R mm 6 mm wcm m o 555555555 w s h .mo 555555555 6 272727272 mwD A WN 272727272 mm 63 863 86 i W .1 63 863 86 22 1111 m m 1111 6H t MVS M MV 0 4 0 .05 1 1 c m 5556 4 511 1 34560992 Pm F. 2 26 9517 m 7 11 A %S 22 I II 6 .5 m m m 789 d 8 68 05 00005 W GS mm m 0 0 9 0 29 f .J3.5. 029504455 Nm 3 W u 075364509 m 2 9888763 NW W C 09998753 I V O m I 1 .LM m m w m m9 6 1 .ln 555555555 fv. B w F 66 555555555 w 272727272 mmD A D .m m 272727272 mm 63 863 86 i. T i 63 863 86 i 2 21111 fl T AA m 22 1111 m t MVS m MV 0L 1 .9 .6 51 H w 1.5 .9 0 024242625 A h 582652224 123 1 L %S 141 4 6 9 U m 83 cow o8 m m We m1 1 e 7 a 055 .5 51 o m wm 0 .2 .60 0 097205375 L u u 046472964 w999852 F w C 9872 m .m n m u m 555555555 w E o 555555555 m 272727272mmD F fi m 272727272mm 43322 F 1 43322 .1 c m E Mm em MVS MV 0505050 e 050 05 52075205050505050 8 s 52 52 5 5 5 5 5 W 222 75443322 m m m22wn 7m4m3w2w w mxxxxxxxxxxxxxxxxx R m nxxx xx xxxxxxxxxxfi 50505050505050505 e l 0 5 22752075443322 .m m fimfi fim wfimfimfim P221111 S P221111 Std. Dev.: 8.4 Std. Dev.: 46.9 Std. Dev.:

TABLE V CHARACTERISTICS OF EFFLUENTS AND CORRESPONDING FLOCCULANTS CHARACTERISTICS FLOCCULANT Description Fraction Trade Manufact- Type Charge Optimum 25 name urer total dos- Wt. age (Ib/t.)* Effluent 99 Super- Cyanamid Polyacryl- Non-ionic 0.20 from blast floc amide furnace clar- I27 ifiers of-a basic oxygen steel making dust,pH: II

SAMPLE 1. Iron Oxide Pounds of flocculant (as commercially supplied) per ton of dry solids in the suspension.

TABLE VI EFFECT OF FLOCCULATION AND CYCLONING ON IRON OXIDE EFFLUENT* Flocculated feed to Reconstituted sample Size Range Unflocculated the cyclone after cycloning (microns) Midpoint Weight Midpoint Weight Midpoint Weight (microns) Cum. Slice (microns) Cum. Slice (microns) Cum. Slice Plus 400 425 100.0 6.5 425 100.0 4.8 350 X 400 375 93.5 5.4 375 95.2 2.0 300 X 350 325 88.1 8.0 325 93.2 6.1 250 X 300 275 80.1 13.2 275 87.1 9.9 150 X 200 225 66.9 17.9 225 77.2 14.7 100 X 150 175 49.0 30.5 175 62.5 20.0 50 X 100 125 18.5 16.5 125 42.5 22.0 45 X 50 75 20.5 17.0 40 X 45 42.5 100.0 0.8 35 X 40 37.5 99.2 0.3 o o 30 X 35 32.5 98.9 0.5 9' 25 X 30 27.5 98.4 0.9 20 X 25 22.5 97.5 2.5 X 17.5 95.0 12.0 or b- 10 X 15 12.5 83.0 23.0 5 X 10 7.5 60.0 42.0 5 2.5 18.0 18.0

Mean: 10.0 Mean: 225.4 Mean: 184.2 Variance: 41.3 Variance: 8050.3 Variance: 9579.6 Std. Dev.: 6.4 Std. Dev.: 89.7 Std. Dev.: 97.9

Origin: Clarifiers of basic oxygen steelmaking dust.

TABLE VI! PERFORMANCE EVALUATION OF 12-lN. CL. C. ON FLOCCULATED Size fraction Fraction Weight Recovery of Overflow recirculated Solids re- Recovery of Total recovery I sohds in the porting to solids in on the underflow (microns) PN slice underflow (U) overflow (0) underflow 0U U OU (2)"X (3) 1 (3)'-" (4) 2nd stage (4) (6) TOTAL 100.0 94.78 5.22 4.37 99.15

We claim; 50 vortex finder so that the flocculant is sprayed into 1. A method of centrifugally separating finely divided the flowing sheet flowing around the rim of the solids from an aqueous suspension thereof, comprising: vortex finder and the flocculant and aqueous susa. feeding the aqueous suspension substantially tan- Penslon are p y and ively interspersed gentially into a cylindrical casing of a blender and and the flocculant 1S adsorbed Onto the wetted i li h aqueous Suspension i a li d i l 55 surfaces of the finely divlded SOlldS without totally open ended vortex finder in the cylindrical casing pp g mdlvldual finely divided solids by the to form, as a thin flowing sheet of the aqueous Pf {molecules and wlthmlt entrapping finely suspension [he finely sglids having d1v1ded SOlldS premature bl'ldgll'lg thfi flOCCU- wetted surfaces from the aqueous medium, a swirlmolecules, i h ll o jn h vortex fi d fl i round 0 maintaining the shape of the swirling hollow cone a rim of the vortex finder, by pressurizing air within the swirling hollow cone 1 Spraying, as a i l spray, a l i f a water with air from a nozzle which is fed within the swirlsgluble molecular weight long chain floccu- 111g hollow cone into the hOllOW center thereof in lant substantially at right angles, with the kinetic the r ly tapering direction of the swirling energies of sprayed flocculant and that of flowing 65 hollow 6006 SO at the kinetic energies of the sheet flowing around the rim of the vortex finder matched, towards a side face of all of the portion of the flowing sheet flowing around the rim of the flocculant and that of the aqueous suspension are not dissipated by eddy currents and fluid masses that would otherwise tend to be in the way of the intersection of the sprayed flocculant and the swirling hollow cone, a, v

the finely divided solids,from'the blend er to a flocculator and flocculating the finely divided-C.

solids, by means ofthe water soluble flocculant adsorbed onto the wetted surfaces, to form dense, strong flocs, which arepredominantly large enough to be substantially separated, from the aqueous medium by a cyclone separator, and feeding the dense, strong flocs and aqueous dium to a cyclone separator and centrifugally separating a major portion of the dense, strong flocs from at least some of the aqueous medium by means of that cyclone separator.

2. A method according to claim 1, wherein after flocculating the finely divided solids, by means of the water soluble flocculant adsorbed onto the wetted surfaces, the flocculated finely divided solids thus formed and the aqueous medium are fed substantially tangentially into the cylindrical casing of a second blender and swirled in a cylindrical, open ended, vortex finder in the cylindrical casing of the second blender, to form, as a thin flowing sheet of the aqueous suspension and flocculated finely divided solids,a swirling hollow cone in the vortex finder of the second blender,fl owing around the rim of that vortex findentafurther solution 7 of a water soluble, high molecular weight, long chain flocculant is sprayed substantiallyfat right angles onto a side face of all of the portion of theflowing sheet of the flocculated finely divided solids in aqueous medium flowing around the rim of the vortex finder in-tthe secnd blender so that further flocculant and aqueous medium are rapidly and intensively interspersed and the flocculant is adsorbed ontothe wetted surfaces of the finely divided solids, maintaining the shape/of the swirling hollow cone in the second blender by pressurizing air within the swirling hollow'cone with air from a nozzle, the finely divided solids are then flocculated a second time, by means of thei further flocculant adsorbed on to the wetted surfaces to form dense, strong flocs which are predominantly large enough to be separated from the aqueous medium by the centrifugal separator, and are then centrifugally separated from some of the aqueous medium b'y th ce'ntrif a'l se arator.

3. A method according to claim 1, wherein flocs of a 12 4. A method according to claim 3, wherein the flocs 'of a size greater than microns are separatedjn two "cyclone separators, with the inlet of the second cyclone separator, receiving the flocs in aqueous medium which a 9 Pe m vsd l mfi le u in th fi s of the two cyclone separators and the dense, strong flocs rom both cycloneseparator s are fed to 'a centrifugal separator and centrifugally separated therein from aqueous medium that has not been removed in the cyclone separators.

5. An apparatusfor centrifugally separating finely divided solids from an aqueous suspension .thereo'f, comprising a blender, a flocculator connected to the outl'etof the blender,: and a cyclone separator connected to the outlet of the flocculator, and wherein the blender comprises a' cylindrical casing with a substantially tangential, aqueous suspension inlet, for directing aqueous suspension fs ubstan ti'ally tangentially into the casing to form a rotating, swirling hollow cone therein, and a substantially tangential outlet, for conveying the rotational 'fio'w substantially tangentially out of the casing andat'a' positionspace'd along the: length of the casing from the inlet, an' 'an n ular w eb sec'ured inside'the casing and sealed thereto and extending radially inwardly "at an; intermediate position between the inlet and outlety a; cylindricalr'open endedg vortex'finder disposed coaxially within the casing and sealed, at an intermediate'position'along its length, 'in the annular web,- the vortex finder extending from each side of ,the annular web jto positions beyond the inlet and outlet, a 'floccula'nt solution nozzle mounted in an end wall of the casing and facing the end of the vortex finder at a position beyondthe outlet, the'floccula'nt solution noz zle being shaped to direct a conical spray of flocculant solution substantially at right angles into the hollow clone of aqueous suspension in the vortex finder, and passing from the inlet to the outle.t,la s it passesaround a rim of the vortex finder, an air nozzle coaxially dis- 40 posed in the vortex finder and facing the flocculant solution nozzle for directing pressurizing air within the swirling hollow cone of theaqueous suspensidn and the conical spray of flocculant, a tube extending along the interior of the vortex finder 'and mounting the "air nozzleto an opposite end wall of the casing to that in which 'the-flocculantnozzle-is mounted,--and asource ofipressurized i air connected. .,to,, theaair, nozzlewthroilight th tube. 

1. A METHOD OF CENTRIFUGALLY SEPARATING FINELY DIVIDED SOLIDS FROM AN AQUEOUS SUSPENSION THEREOF, COMPRISING: A. FEEDING THE AQUEOUS SUSPENSION SUBSTANTIALLY TANGENTIALLY INTO A CYLINDRICAL CASING OF A BLENDER AND SWIRLING THE AQUEOUS SUSPENSION IN A CYLINDRICAL, OPEN ENDED VORTEX FINDER IN THE CYLINDRICAL CASING TO FORM, AS A THIN FLOWING SHEET OF THE AQUEOUS SUSPENSION WITH THE FINELY DIVIDED SOLIDS HAVING WETTED SURFACES FROM THE AQUEOUS MEDIUM, A SWIRLING HOLLOW CONE IN THE VORTEX FINDER FLOWING AROUND A RIM OF THE VOTEX FINDER, B. SPRAYING, AS A CONICAL SPRAY, A SOLUTION OF A WATER SOLUBLE HIGH MOLECULAR WEIGHT LONG CHAIN FLOCCULANT SUBSTANTIALLY AT RIGHT ANGLES, WITH THE KINETIC ENERGIES OF SPRAYED FLOCCULANT AND THAT OF FLOWING SHEET FLOWING AROUND THE RIM OF THE VORTEX FINDER MATCHED, TOWARDS A SIDE FACE OF ALL OF THE PORTION OF THE FLOWING SHEET FLOWING AROUND THE RIM OF THE VORTEX FINDER SO THAT THE FLUCCULANT IS SPRAYED INTO THE FLOWING SHEET FLOWING AROUND THE RIM OF THE VOTEX FINDER AND THE FLUOCCULANT AND AQUEOUS SUSPENSION ARE RAPIDLY AND INTENSIVELY INTERSPERSED AND THE FLOCCULANT IS ADSORBED ONTO THE WETTED SURFACES OF THE FINELY DIVIDED SOLIDS WITHOUT TOTALLY WRAPPING INDIVIDUAL FINELY DIVIDED SOLIDS BY THE FLOCCULANT MOLECULES AND WITHOUT ENTRAPPING FINELY DIVIDED SOLIDS BY PREMATURE BRIDGING BY THE FLOCCULANT MOLECULES, C. MAINTAINING THE SHAPE OF THE SWIRLING HOLLOW CONE BY PRESSURIZING AIR WITHIN THE SWIRLING HOLLOW CONE WITH AIR FROM A NOZZLE WHICH IS FED WITHIN THE SWIRLING HOLLOW CONE INTO THE HOLLOW CENTER THEREOF IN THE OUTWARDLY TAPERING DIRECTION OF THE SWIRLING HOLLOW CONE SO THAT THE KINETIC ENERGIES OF THE FLOCCULANT AND THAT OF THE AQUEOUS SUSPENSION ARE NOT DISSIPATED BY EDDY CURRENTS AND FLUID MASSES THAT WOULD OTHERWISE TEND TO BE IN THE WAY OF THE INTERSECTION OF THE SPRAYED FLOCCULANT AND THE SWIRLING HOLLOW CONE, D. FEEDING THE AQUEOUS SUSPENSION, WITH THE SAID FLOCCULANT ADSORBED ONTO THE SAID WETTED SURFACES OF THE FINELY DIVIDED SOLIDS, FROM THE BLENDER TO A FLOCCULATOR AND FLOCCULATING THE FINELY DIVIDED SOLIDS, BY MEANS OF THE WATER SOLUBLE FLOCCULANT ADSORBED ONTO THE WETTED SURFACES, SO FROM DENSE, STRONG FLOCS, WHICH ARE PREDOMINANTLY LARGE ENOUGH TO BE SUBSTANTIALLY SEPARATED FROM THE AQUEOUS MEDIUM BY A CYCLONE SEPARATOR, AND E. FEEDING THE DENSE, STRONG FLOCS AND AQUEOUS MEDIUM TO A CYCLONE SEPARATOR AND CENTRIFUGALLY SEPARATING A MAJOR PORTION OF THE DENSE, STRONG FLOCS FROM AT LEAST SOME OF THE AQUEOUS MEDIUM BY MEANS OF THAT CYCLONE SEPARATOR.
 2. A method according to claim 1, wherein after flocculating the finely divided solids, by means of the water soluble flocculant adsorbeD onto the wetted surfaces, the flocculated finely divided solids thus formed and the aqueous medium are fed substantially tangentially into the cylindrical casing of a second blender and swirled in a cylindrical, open ended, vortex finder in the cylindrical casing of the second blender, to form, as a thin flowing sheet of the aqueous suspension and flocculated finely divided solids, a swirling hollow cone in the vortex finder of the second blender, flowing around the rim of that vortex finder, a further solution of a water soluble, high molecular weight, long chain flocculant is sprayed substantially at right angles onto a side face of all of the portion of the flowing sheet of the flocculated finely divided solids in aqueous medium flowing around the rim of the vortex finder in the second blender so that further flocculant and aqueous medium are rapidly and intensively interspersed and the flocculant is adsorbed onto the wetted surfaces of the finely divided solids, maintaining the shape of the swirling hollow cone in the second blender by pressurizing air within the swirling hollow cone with air from a nozzle, the finely divided solids are then flocculated a second time, by means of the further flocculant adsorbed on to the wetted surfaces to form dense, strong flocs which are predominantly large enough to be separated from the aqueous medium by the centrifugal separator, and are then centrifugally separated from some of the aqueous medium by the centrifugal separator.
 3. A method according to claim 1, wherein flocs of a size greater than 10 microns are centrifugally separated from the aqueous medium by the cyclone separator.
 4. A method according to claim 3, wherein the flocs of a size greater than 10 microns are separated in two cyclone separators, with the inlet of the second cyclone separator receiving the flocs in aqueous medium which has not been removed from the effluent in the first of the two cyclone separators and the dense, strong flocs from both cyclone separators are fed to a centrifugal separator and centrifugally separated therein from aqueous medium that has not been removed in the cyclone separators.
 5. An apparatus for centrifugally separating finely divided solids from an aqueous suspension thereof, comprising a blender, a flocculator connected to the outlet of the blender, and a cyclone separator connected to the outlet of the flocculator, and wherein the blender comprises a cylindrical casing with a substantially tangential, aqueous suspension inlet, for directing aqueous suspension substantially tangentially into the casing to form a rotating, swirling hollow cone therein, and a substantially tangential outlet, for conveying the rotational flow substantially tangentially out of the casing and at a position spaced along the length of the casing from the inlet, an annular web secured inside the casing and sealed thereto and extending radially inwardly at an intermediate position between the inlet and outlet, a cylindrical, open ended, vortex finder disposed coaxially within the casing and sealed, at an intermediate position along its length, in the annular web, the vortex finder extending from each side of the annular web to positions beyond the inlet and outlet, a flocculant solution nozzle mounted in an end wall of the casing and facing the end of the vortex finder at a position beyond the outlet, the flocculant solution nozzle being shaped to direct a conical spray of flocculant solution substantially at right angles into the hollow cone of aqueous suspension in the vortex finder, and passing from the inlet to the outlet, as it passes around a rim of the vortex finder, an air nozzle coaxially disposed in the vortex finder and facing the flocculant solution nozzle for directing pressurizing air within the swirling hollow cone of the aqueous suspension and the conical spray of flocculant, a tube extending along the interior of the vortex finder and mounting the air nozzle to an opposite end wall of the casing to that in whiCh the flocculant nozzle is mounted, and a source of pressurized air connected to the air nozzle through the tube. 